Tell me I'm wrong

[Danger]

I was talking to a guy who didn't seem to know a lot about poker but like to think he does. (You know the type) Well, we started talking about odds and he said you have to consider the other peoples cards(unseen to you) when figuring out your odds. Well this goes against everything I've ever read. I kind of wrote him off but later I was thinking about it and something wasn't setting right with me. So I tried to work out the math and this is the example I worked out.

Say you have a flush draw on the flop with two cards to the flush in your hand. Normally, your looking at 9 outs and 47 unseen cards which works out to about a 19% chance of hitting your flush on the turn. But here is what I tried.

You have 2 cards out of the 13 suited cards in your hand. (13-2=11)
And 2 cards out of the deck. (52-2=50)
If on average the rest of the suited card are distributed evenly through out the deck and there are 9 other players at the table then on average 4 of your suit will have been dealt out. (11/50*9*2 = 3.96 ~ 4)
11 of your suit, 50 cards in the deck, 9 players, 2 cards each.
So now 3.96 gets subtracted from the 11 and now there is only 7.04 of your suit, but there is also 18 more cards off the deck. So to update: 7.04 of your suit and 32 (50-18 = 32) left in the deck. Now the flop comes and there's your flush draw. 2 more cards get subtracted from your suit and 3 from the deck. Update: 5.04 of your suit and 29 left in the deck. Now your chance of making the flush on the turn is 5.04/29 = 17%

Now, I know what your thinking. All that work for a 2% difference. And I agree, you still should be looking for about 4 to 1 on your money for ok pot odds, but there still seems to be a discrepancy in the numbers. What do you think?